1. Field of the Invention
The present invention relates to a transmission device and a redundant configuration between the transmission device and a layer 2 switch.
2. Description of the Related Art
With recent high speed data communication in wide area networks (WAN), demand for services with layer 2, which is low cost and easy to manage, is increasing. For the layer 2 service, a layer 2 switch is used (for example, Japanese Patent Laid-Open Publication No. 2003-18196). The layer 2 switch is constituted by an input/output port, a media access control (MAC) processing unit, a switch processing unit, and a MAC address table. The layer 2 switch has a virtual bridged local-area-network (VLAN) function and a learn function for learning information (MAC address) about layer 2 (data link layer), and makes determination on frame transfer processing.
In a switching hub, which is commonly used as a layer 2 switch, the input/output port is connected to a WAN or a host, and data is communicated there between. The MAC address processing unit extracts a destination MAC address from a packet that is received through the input/output port, and sends the destination MAC address to the MAC address table. The MAC address table specifies an input/output port through which data is output based on the destination MAC address. The MAC address processing unit sends the packet to the switch processing unit and causes the switch processing unit to output the packet from the specified input/output port.
A layer 2 switch used for wide-area layer 2 network services that are provided mainly by communication carriers is connected to a terminal device or the like of a subscriber through a transmission path at one side, and to a wide-area layer 2 network through a transmission device at the other side of an input/output port. The layer 2 switch and the transmission device are installed at one station. Other stations have the same configuration. When connection is established between distant places through a transmission device in this manner, measures against failure, such as disconnection of the transmission path, are required, to maintain stability and reliability of the services.
A technique has been proposed in which more than one operating path are provided between network devices, and a different route is used when failure occurs in one operating path (for example, Japanese Patent Laid-Open Publication No. 2002-26956). Moreover, a technique has been proposed in which a MAC address table is managed, and when inconsistency with an actual connection state is found in the MAC address table due to failure in connection with an external device or the like, the MAC address table is appropriately updated to prevent network load increases caused by retransmission of packets and the like (for example, Japanese Patent Laid-Open Publication No. 2000-151674).
FIG. 12 is a schematic diagram illustrating a conventional redundant configuration using plural layer 2 switches. The redundant configuration shown in FIG. 12 is an original configuration by a layer 2 switch manufacturer. Layer 2 switches 53 and 54 are connected to a plurality of ports 51a and 51b of a transmission device 50, respectively. The layer 2 switch 53 is used as a master and the layer 2 switch 54 is used as a slave, and the redundant configuration with two transmission paths 55a and 55b is established. These master/slave layer 2 switches 53 and 54 are connected to a single layer 2 switch 56 to be connected to a subscriber. The layer 2 switch 56 has a hot standby function, and enables switching of transmission paths to the transmission path 55b if transmission path is disconnected due to failure occurred at a point X on the transmission path 55b. 
FIG. 13 is a schematic diagram for explaining link aggregation by the layer 2 switch. The link aggregation is defined by IEEE 802.1ad, and is a function of obtaining a line speed faster than the physical speed of a single line by using more than one transmission path. Between two ports of the transmission device 50 and two ports of the layer 2 switch 53, two lines of the transmission paths 55 (55a, 55b) are connected. With this arrangement, a line speed twice as fast (for example, 2 GBit/sec) as the physical line speed of one transmission path (for example, 1 GBit/sec) can be obtained.
However, the layer 2 switch currently used by communication carriers for the layer 2 services is not equipped with a redundancy function of the transmission path to the transmission device. Therefore, if failure occurs in the transmission path between the transmission device and the layer 2 switch disconnecting the transmission line, service is stopped at once.
If it is configured to switch the disconnected transmission route to another route as the technique disclosed in Japanese Patent Laid-Open Publication No. 2002-26956, more than one transmission device and more than one wide area transmission path are required, resulting in increased installation cost and complicated operation and maintenance. In addition, the network devices must communicate control packets to monitor the condition of each other using a port of the network device that is on standby, and a function for performing a control process on the communicated control packets must be prepared in both of the network devices.
Furthermore, in the configuration shown in FIG. 12, three layer 2 switches 53, 54, and 56 are required. Since the number of devices increases, the installation cost increases. Moreover, when the transmission path is disconnected due to failure on the transmission path 55a, it is necessary to send disconnection information i to a transmission device 58 installed in a receiver station through a relay transmission path 57. Therefore, route switching within a section (station) cannot be performed.
If the route switch within a section cannot be performed, cost and labor for operation management increase due to the complicated network. FIG. 14 is a schematic of route switching when failure occurs. The configuration of the transmission device is omitted therein. A center station includes the master and slave layer 2 switches 53 and 54. When a plurality of terminal stations 60 (60a, 60b, . . . , 60n) are connected to this center station, each of the terminal stations 60 is required to be connected to both of the master and slave layer 2 switches 53 and 54 through the transmission paths 57. Thus, the number of the transmission paths 57 increases, complicating the network structure. For example, the terminal station 60a must be connected to the master and slave layer 2 switches 54 and 54 using two transmission paths 57a and 57c. 
In addition, when the transmission 55a of the master layer 2 switch 53 is disconnected due to failure at the point X and the like, it is necessary to switch, in the center station, from the master to the slave, and further, it is necessary to switch from the master transmission path 57a to the slave transmission path 57b. Furthermore, if such failure occurs in the center station, the entire system in the transmission path 57 must be switched from the master to the slave. The center station must inform the switching of the transmission path 57 and the switching from the master to the slave to each of the terminal stations 60, complicating operation management.
Moreover, since the link aggregation function shown in FIG. 13 uses two lines of transmission paths, if one of the transmission paths 55, for example, the transmission path 55a, is disconnected due to failure, the communication speed is reduced by half. Therefore, it becomes impossible to obtain the prescribed line speed. Furthermore, since two ports in the layer 2 switch 53 to which the transmission path 55 is connected uses a single interface card 53a, if this interface card 53a breaks, the communication service is stopped at once.
As described above, from the view points of operation management and cost, a redundant configuration between a transmission device and a layer 2 switch is demanded such that the configuration is applicable to both Gigabit Ethernet (GbE) (registered trademark) and Fast Ethernet (FE) (registered trademark), and switching of routes between the transmission device and the layer 2 switch is not affected by the switching of other transmission paths, for example, relay transmission paths.